**4.1 Equipment**

80 Biogas

Anaerobic digestion (AD) is an attractive treatment for this waste of difficult disposal. AD processes transform the organic matter contained in a certain waste in biogas as main product. This process is carried out for different kind of microorganisms which work in a

Anaerobic treatment of moderate and high strength wastes with high biodegradable content presents a number of advantages in comparison to the classical aerobic processes: a) quite a high degree of purification with high-organic load feeds can be achieved; b) low nutrient requirements are necessary; c) small quantities of excess sludge are usually produced and finally, d) a combustible biogas is generated. The production of biogas enables the process to generate or recover energy instead of just energy-saving; this can reduce operational costs as compared with other processes such as physical, physico-chemical or biological aerobic

Previous works carried out at pilot-scale have shown that most of agro-industrial residues, such as sugar beet pulp, potato pulp, potato thick stillage and brewer´s grains, can be treated anaerobically with an efficient solids stabilisation and energy recovery, if the applied process-type (one or two stages) is selected according to the C:N ratio of the residues. These works demonstrated that at hydraulic retention times (HRT) of between 10 and 20 days, normally, the 50-60% of the organic matter was degraded. The ultimate anaerobic biodegradability was higher and lied between 76% (brewer´s grain) and 88% (potato pulp), which demonstrated that more than 60% of the available energy potential could be used in the industrial processes. The gas production varied between 300 and 500 m3 biogas per ton of dry matter with a methane content of 60-70%. The undigested solids, which were separated from the effluent of the reactors could be completely stabilised after a short

A number of kinetic models have been proposed for the process of anaerobic digestion. Early models were based on a single-culture system and used the Monod equation or variations. More recently, several dynamic simulation models have been developed based on a continuous multi-culture system; these correspond to the major bioconversion steps in anaerobic digestion but again make the assumption that culture growth obeys Monod type kinetics. Doubt has been expressed by several investigators on the validity of applying the Monod equation to waste treatment as the specific growth rate is expressed only as a function of the concentration of the limiting substrate in the reactor. The Monod equation contains no term relating to input substrate concentration; this implies that the effluent substrate concentration is independent of the input concentration. Experimental results do not always agree with this implication; for example the anaerobic digestion of dairy manure, beef cattle manure at mesophilic and thermophilic temperatures, rice straw or poultry litter

Deviation from the Monod relationship in many digestion systems may be due to their complexity. This complexity has necessitated the use of generalized measures of feed and effluent strength, namely total Chemical Oxygen Demand (COD) and volatile solids (VS), which may not truly reflect the nature of the growth-limiting substrate. Utilizable carbon in the digester is derived from the hydrolysis of polymeric compounds, constituting the waste, by exo-enzymes in the extracellular medium or on the surface /vicinity of the

aerobic post-treatment to be used as a soil conditioner (Borja et al., 2006).

**3. Anaerobic digestion as an alternative for treatment of two-phase OMSW** 

coordinate and interdependent chain until biogas obtaining.

treatments (Borja et al., 2006).

(Borja et al., 2003).

An anaerobic reactor with a working volume of 1 litre equipped with magnetic stirring and placed in a thermostatic chamber at 35 ºC was used. The reactor had an upper settling zone designed to minimize loss of the biomass responsible for the process. The reactor was fed daily by means of an external feeder and liquid effluent removed daily through a hydraulic seal, comprising 25 cm liquid column, designed to prevent air from entering the reactor and biogas from leaving. This reactor has been described in detail elsewhere (Martín et al., 1991).

Influence of Substrate Concentration on the Anaerobic

HRTs of 50.0, 25.0, 16.6, 12.5 and 10.0 d, respectively.

a duration of 2-3 times the corresponding HRT.

American Public Health Association (APHA, 1989).

ensure that representative data were obtained.

**5. Results and discussion** 

**operational parameters** 

**4.4 Experimental procedure** 

between 46 and 60 d.

loadings.

**4.5 Chemical analyses** 

Degradability of Two-Phase Olive Mill Solid Waste: A Kinetic Evaluation 83

The anaerobic reactor was initially charged with 300 mL of distilled water, 500 mL of the inoculum and 200 mL of a nutrient-trace element solution. The composition of this nutrient-

The start-up of the reactor involved stepped increases in COD loading using an influent substrate concentration of 17.2 g COD/L. During this period the organic loading rate (OLR) was gradually increased from 0.25 to 0.50 g COD/(L d) between 1 and 15 d, 0.75 g COD/(L d) between 16 and 30 d, 1.00 g COD/(L d) between 31 and 45 d and finally 1.25 g COD/(L d)

After the preliminary step, the reactor was fed in series of semicontinuous experiments using OLRs of 0.9, 1.2, 1.4, 1.7, 2.1, 2.8, 3.5, 4.1 L COD/(L d) for the OWSW1, which correspond to hydraulic retention times (HRTs) of 40.0, 28.6, 25.0, 20.0, 16.6, 12.5, 10.0 and 8.3 d, respectively. After these experiments with OMSW 1 five different OLRs were assessed for the OMSW 2, 3.0, 6.0, 9.05, 12.0 and 15.0 g COD/(L d), these OLRs corresponded to

Once steady-state conditions were achieved at each feed flow-rate, the daily volume of methane produced, and total and soluble COD, pH, total volatile fatty acids (TVFA) and volatile solids (VS) of the different effluents were determined. The samples were collected and analysed for at least 5 consecutive days. The steady-state value of a given parameter was taken as the average of these consecutive measurements for that parameter when the deviations between the observed values were less than 3% in all cases. Each experiment had

The organic loadings applied in this work were increased in a stepwise fashion in order to minimise the transient impact on the reactor that might be induced by a sudden increase in

The following parameters were determined: total and soluble COD, pH, total solids, mineral solids, volatile solids, total suspended solids, mineral suspended solids, volatile suspended solids, total volatile fatty acids (TVFA), alkalinity and total phenolic compounds. All analyses were carried out according to the recommendations of the Standard Methods of

In each steady-state experiment, samples were collected and the above parameters analysed. The pH and gas volume were determined daily, whilst the remaining parameters were measured at least five times per week on five different samples taken on different days to

**5.1 Influence of substrate concentration and OLR on the COD removal efficiency and** 

The anaerobic degradability studies were carried out using two different two-phase OMSWs with COD concentrations of 35 g COD/L (OMSW 1) and 150 g COD/L (OMSW 2). The

trace element solution is given in detail elsewhere (Borja et al., 2001).

The methane volume produced in the process was measured using a 5 litre Mariotte reservoir fitted to the reactor. A tightly closed bubbler containing a NaOH solution (3 M) to collect the CO2 produced in the process was intercalated between the two elements. The methane produced displaced a given volume of water from the reservoir, allowing ready determination of the gas (Martín et al., 1991).
